1. Field of the Invention
The present invention relates to a solid-state imaging device having a so-called back-illuminated structure for illuminating light on a light-receiving sensor portion from the back side opposite to electrodes and interconnections and a method of manufacturing such a back-illuminated type solid-state imaging device.
2. Description of the Related Art
It has been customary that a solid-state imaging device has a surface-illuminated type structure with electrodes and interconnections formed on a substrate to illuminate light on a light-receiving sensor portion from above the electrodes and the interconnections.
FIG. 1 of the accompanying drawings is a schematic cross-sectional view showing a solid-state imaging device having a surface-illuminated type structure.
A solid-state imaging device, generally depicted by the reference numeral 50 in FIG. 1, is a CMOS (complementary metal-oxide semiconductor) solid-state imaging device having a surface illuminated type structure.
As shown in FIG. 1, a photodiode PD comprising a light-receiving sensor portion of each pixel is formed within a silicon substrate 51, and an interconnection layer 53 of a multi-layer is formed on the silicon substrate 51 through an interlayer insulator 52. Further, a color filter 54 and a lens 55 are formed on the layer above the interconnection layer 53.
Light L is passed from the lens 55 through the interlayer insulator 52 between the color filter 54 and the interconnection layer 53 and introduced into the photodiode PD of the light-receiving sensor portion.
However, as the solid-state imaging device is microminiaturized increasingly, the pitch of interconnection becomes narrower and the interconnection layer 53 is formed of many more layers so that a distance between the lens 55 and the photodiode PD of the light-receiving sensor portion is increased unavoidably.
As a consequence, as shown hatched in FIG. 1, a part Lx of the light L, which became incident on the solid-state imaging device 50 obliquely, is interrupted by the interconnection layer 53 and cannot be introduced into the photodiode PD, thereby causing a bad phenomenon such as shading to occur.
As a plan for improving the above bad phenomenon, a back-illuminated type solid-state imaging device has been proposed to illuminate light on a light-receiving sensor portion from the opposite side of the surface on which interconnections are formed (for example, see Cited patent reference 1).
While the cited patent reference 1 has described a CCD (charge-coupled device) solid-state imaging device having a back-illuminated type structure, it is considered that this back-illuminated type structure can be applied to the CMOS type solid-state imaging device.
FIG. 2 is a schematic cross-sectional view showing an example of a CMOS type solid-state imaging device with a back-illuminated type structure being applied thereto.
As shown in FIG. 2, a photodiode PD comprising a light-receiving sensor portion of each pixel is formed within a single crystal silicon layer 61, and a color filter 64 and a lens 65 are formed on the single crystal silicon layer 61. The single crystal silicon layer 61 is formed of a silicon substrate whose thickness is reduced, as will be described later on.
On the other hand, a multilayer interconnection layer 63 is formed under the single crystal silicon layer 61 through an interlayer insulator 62, and the interlayer insulator 62 in which the interconnection layer 63 is formed is supported by a supporting substrate 66 formed under the interconnection layer 63.
Then, light L is introduced from the lens 65 into the photodiode PD of the light-receiving sensor portion formed on the single crystal silicon layer 61.
Assuming that the side in which the interconnection layer 63 is formed is the surface side, the light L is introduced into the photodiode PD from the back side. Thus, this structure of the above CMOS type solid-state imaging device is referred to as a “back-illuminated type structure”.
In the back-illuminated type solid-state imaging device like the solid-state imaging device 60 shown in FIG. 2, the incident light is not interrupted by the interconnection layer 63 so that an effective vignetting factor relative to oblique incident light can reach 100%.
Accordingly, it can be expected much that sensitivity of this back-illuminated type solid-state imaging device can be improved considerably and that this back-illuminated type solid-state imaging device will be free from the shading.
The solid-state imaging device 60 shown in FIG. 2 can be manufactured as follows.
First, the photodiode PD 60 comprising the light-receiving sensor portion is formed near the surface of a silicon substrate 71 by a suitable method such as ion implantation. Then, an interconnection layer 63A of a first layer is formed on the silicon substrate 71 through a gate insulating film 72 and the interconnection layer 63 following the second layer is formed through the interlayer insulator 62, in that order (see FIG. 3A).
Then, as shown in FIG. 3B, an SiO2 layer 73 is deposited on the surface of the insulating layer 62 and the surface thereof is polished. At the same time, a silicon substrate is prepared as the supporting substrate 66 and an SiO2 layer 74 is formed on the surface of the supporting substrate 66. These SiO2 layers 73, 74 are bonded together in an opposing fashion.
Subsequently, as shown in FIG. 3C, the resultant product is inverted up and down so that the side of the supporting substrate 66 may be directed to the lower direction.
Next, as shown in FIG. 3D, the silicon substrate 71 is reduced in thickness by polishing the surface of the silicon substrate 71 and thereby the single crystal silicon layer 61 in which the photodiode PD is formed and which has a predetermined thickness is obtained.
Next, as shown in FIG. 3E, the color filter 64 and the lens 65 are formed on the single crystal silicon layer 61 through a planarized layer 75 (not shown in FIG. 2).
Thereafter, the supporting substrate 66 is reduced in thickness according to the need.
In this manner, it is possible to manufacture the solid-state imaging device 60 shown in FIG. 2.
[Cited patent reference 1]: Official gazette of Japanese laid-open patent application No. 6-283702 (FIGS. 5A, 5B) While the lens 65 is formed on the single crystal silicon layer 61 in the final process (FIG. 3E) of the manufacturing processes shown in FIGS. 3A to 3E, at that time, the lens 65 has to be formed in alignment with the photodiode PD that has already been formed so that an alignment mark is indispensable to such final process (FIG. 3E).
However, in the structure of the back-illuminated type solid-state imaging device according to the related art, it has not been considered so far to form an alignment mark.
Also, in the back-illuminated type solid-state imaging device, since the lens is formed on the back side of the silicon substrate, it is not possible to manufacture the alignment mark of the lens relative to the photodiode by a method similar to the alignment mark producing method in the surface-illuminated type solid-state imaging device (method of forming the alignment mark on the surface of the silicon substrate).
Also, similarly, since the supporting substrate and the like are attached to the side of the interconnection layer, it is impossible to form a pad contact by an ordinary method.
For this reason, the alignment mark forming method and the pad contact forming method should be established. Otherwise, it becomes impossible to realize a back-illuminated type solid-state imaging device, and hence it becomes difficult to microminiaturize the solid-state imaging device and to improve performance such as resolution.
That is, in order to realize the back-illuminated type solid-state imaging device, there are required an alignment mark forming method for forming an alignment mark on a silicon substrate in order to align the photodiode and the lens and a pad contact forming method.